Many power generation plants produce electricity by converting energy (e.g. fossil fuel, nuclear fusion, hydraulic head and geothermal heat) into mechanical energy (e.g. rotation of a turbine shaft), and then converting the mechanical energy into electrical energy (e.g. by the principles of electromagnetic induction).
Some of these power generation plants, such as a fossil fuel power generation plant, comprise a turbine and a generator. The turbine converts fossil fuel energy into mechanical energy in the form of turbine shaft rotation through a steam or combustion cycle. In a steam cycle, fuel (e.g. coal) is burned in a boiler to produce a steam force that is introduced into a steam turbine. The steam force works to turn stages of airfoil blades that are attached to and rotate a shaft. Corresponding stages of stationary airfoil vanes help direct the steam force over the blades. In a combustion cycle, compressed air and fuel (e.g. oil or natural gas) are mixed and burned in a combustion section of a combustion turbine to produce a combustion force that works to turn the stages of airfoil blades. In either cycle, fossil fuel energy is ultimately converted into mechanical energy in the form of turbine shaft rotation. It is known to use both a steam cycle and a combustion cycle to increase power generation plant efficiency in what is commonly termed a combined cycle power generator plant. Such combined cycle power generator plants are described in U.S. Pat. Nos. 4,932,204, 5,255,505, 5,357,746, 5,431,007, 5,697,208 and 6,145,295, each of which is hereby incorporated by reference in their entirety.
One aspect of the above-described power generation scheme involves the cooling of turbine airfoil blades and vanes. In order to maximize power generation plant efficiency, gas turbine inlet temperatures can attain temperatures of about 2600° F. or higher. These high temperatures, however, can melt or otherwise harm the turbine airfoils, especially those in the first stages. A coolant is therefore used to inhibit airfoil melting, cracking, creeping, oxidizing or other failure by maintaining the airfoil temperature at about 1700–2000° F. or less. The cooling scheme is advantageously incorporated into the airfoil configuration itself.
Turbine airfoils are typically cooled through one of two types of cooling schemes, commonly termed open loop and closed loop. An open loop scheme is generally used in a combustion cycle due to the ready availability of air. In an open loop scheme, compressed air is bled from the compressor section of the combustion turbine. The compressed air is directed through inlet passages of an airfoil within the combustion section of the combustion turbine, and then into the airfoil cavity. This cooling air then travels from the airfoil cavity, along a cooling passage, and exits the airfoil via outlet passages. The outlet passages direct the cooling air along the exterior wall of the airfoil. By this configuration, the airflow cools the airfoil interior by impingement and convection currents and cools the airfoil exterior by film flow.
A disadvantage of this open loop cooling scheme, however, is that extracting coolant air from the compressor section causes parasitic losses to the thermodynamic efficiency of the power generation plant. Another disadvantage of open loop cooling is that air has a relatively low latent specific heat and is therefore relatively inefficient at absorbing heat to thereby cool the airfoil.
A closed loop cooling scheme can be used to overcome several disadvantages of open loop cooling. A closed loop scheme is generally used in a steam cycle due to the ready availability of steam. In closed loop cooling, steam from the steam turbine and/or a heat recovery steam generator (HRSG) is directed through inlet passages of an airfoil within the steam turbine, and then into the airfoil cavity. This cooling steam then circulates from the airfoil cavity, along a cooling passage, and then back into the airfoil cavity. The now warmed used coolant steam is then removed from the cavity and replaced with new coolant steam.
Although a closed loop scheme is generally preferable to an open loop scheme because steam has a higher latent specific heat than air, one disadvantage of closed loop cooling is that is the steam must be provided at a relatively high pressure (about 500–1000 psi, which is about 3–5 times greater than the air pressure used in an open loop system). This high pressure, as well as thermal stresses, place severe stresses on the airfoils and require that the airfoils have a relatively strong construction. Also, it is difficult and expensive to manufacture a suitably strong thin walled airfoil. It has been thus been found useful to use an airfoil having internal ribs to provide relative strength and assist in cooling.
Conventional steam cooled airfoils having internal cooling passages are typically made by welding discrete perforated inserts between the perimeter wall of the airfoil cavity and the exterior wall of the airfoil. The perforated inserts have a dimension that maintains a distance between the airfoil cavity and the airfoil exterior wall so that coolant steam can pass through the airfoil cavity, through the perforated insert, and then back into the airfoil cavity to provide impingement cooling. The perforated inserts are typically machined by steel rolling, which can be difficult and expensive. Moreover, this approach exceeds the available steam pressure drop and generates degraded impingement HTCs due to inherent crossflow effects.
There is thus a need for an improved airfoil cooling scheme. There is also a need for an airfoil that can be cooled in an improved manner. There is a further need for an improved process for manufacturing an airfoil that requires cooling. There is also a need for a thin walled pin array cast airfoil that is cooled through a closed loop steam cooling scheme which is located in the first stages of a combustion turbine within a combined cycle power generation plant.